EP4596129A1

EP4596129A1 - Steel plate

Info

Publication number: EP4596129A1
Application number: EP23872641.8A
Authority: EP
Inventors: Nobuyuki Yoshimura; Ryotaro SHIRAISHI; Ryuichi Honma; Fumitoshi Takamine; Hironori Wakamatsu; Takeshi Okubo
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2022-09-30
Filing date: 2023-09-29
Publication date: 2025-08-06
Also published as: JPWO2024071422A1; WO2024071422A1; EP4596129A4; JP7469734B1; WO2024071422A9; KR20250051771A; CN119968473A

Abstract

A steel plate has a predetermined chemical composition, and has an α value of 1.00 to 1.50 mass%, a β value of 10.0 to 15.0, a y value of 0.70 to 1.50 mass%, a Ceq value of 0.550 to 0.620 mass%, a yield strength of 670 to 870 N/mm², a tensile strength of 780 to 940 N/mm², and a Charpy absorbed energy at -65°C of 100 J or more, in measurement of a hardness distribution at a pitch of 0.05 mm in 1 mm × 1 mm at a 1/4 thickness position, an average value of hardness at 121 measurement positions is 265 Hv to 290 Hv and a standard deviation is 20 or less, and a plate thickness is 10 to 60 mm.

Description

TECHNICAL FIELD

The present invention relates to a steel plate.
Priority is claimed on Japanese Patent Application No. 2022-157410, filed September 30, 2022 , the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, there has been a strong demand for reducing greenhouse gases as a measure for climate change, and carbon dioxide capture and storage (CCS), which is a technology for capturing and storing carbon dioxide (hereinafter, referred to as CO₂), has been attracting attention as a technology for realizing carbon neutrality. In CCS, CO₂ discharged from a CO₂ discharge source, such as a refinery, a power plant, or a chemical plant, is separated and captured, and is then pressed and stored in a deep underground reservoir. In a case where a capture facility for separating and capturing CO₂ and a storage facility for pressing and storing CO₂ in an underground reservoir are spatially separated from each other, it is necessary to transport the separated and captured CO₂ between the facilities via a pipeline, ship, or the like.
In CO₂ transportation by ship, CO₂ is transported by being liquefied and filled into a transportation tank provided in the ship. Therefore, the CO₂ transportation efficiency is improved. However, in order to prevent CO₂ from being solidified (converted into dry ice) in the transportation tank, it is necessary to transport CO₂ while holding it at a pressure of about 2 MPa. In addition, it is necessary to keep CO₂ at about -35°C in order to maintain CO₂ in a liquid state at a pressure of about 2 MPa. Furthermore, in order to reduce the weight of the ship, there is also a demand for increasing the strength of a steel plate used, and thus making the thickness of the transportation tank as small as possible.
Accordingly, the steel plate used as a material of the transportation tank is required to have a high strength and excellent low-temperature toughness. For example, the steel plate is required to have, as the strength, a tensile strength of 780 N/mm² or more. In addition, although also depending on the plate thickness, the steel plate is required to have, as the low-temperature toughness, excellent low-temperature toughness evaluated by a Charpy test under the most severe condition of -65°C, in a case where the plate thickness is 20 to 60 mm, which are used for the transportation tank. Here, the reason the test temperature is -65°C in the Charpy test is that the Charpy test is a small-sized test and the evaluation is usually performed at a temperature that is a certain degree lower than the general service temperature according to the plate thickness.
Furthermore, in a large-sized welded structure such as a transportation tank, stress relieving annealing may be performed on a welded part to further reduce the possibility of the occurrence of fracture. Stress relieving annealing is a heat treatment method in which a welded part of a structure after welding is heated to a temperature of an Ac1 transformation point or lower, and then slowly cooled to reduce the residual stress generated by welding. However, when stress relieving annealing is applied to a high-tensile-strength steel having a tensile strength of 780 N/mm² or more, alloy carbides selectively precipitate at crystal grain boundaries and cause intergranular embrittlement, so that the toughness of the part subjected to the stress relieving annealing extremely decreases. Generally, this phenomenon is called stress relieving (SR) embrittlement. In particular, in a high-tensile-strength steel containing B and produced by quenching and tempering, there is a strong tendency for SR embrittlement to occur. In such a high-tensile-strength steel, the base plate becomes embrittled, and the welded heat-affected zone obtained in a case where a welded joint is created using the high-tensile-strength steel also becomes significantly embrittled.
Therefore, in order to ensure high safety in a transportation tank produced using the high-tensile-strength steel, the base plate and the welded part (particularly, the welded heat-affected zone) preferably have excellent low-temperature toughness even when subjected to stress relieving annealing.
From the above-described viewpoint, several technical proposals have been made in the related art. For example, Patent Document 1 discloses a high-strength steel plate in which a chemical composition is adjusted and an average grain size is set to 15 µm or less. However, the steel plate described in Patent Document 1 has not been evaluated for low-temperature toughness at -65°C and there is room for further improvement in low-temperature toughness.

Citation List

Patent Document

Patent Document 1: Japanese Patent No. 5590271

SUMMARY OF INVENTION

Technical Problem

The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel plate suitable for a liquefied CO₂ transportation tank that has excellent strength of a base plate and low-temperature toughness of the base plate and a welded heat-affected zone and also has excellent strength of the base plate and low-temperature toughness of the base plate and the welded heat-affected zone after stress relieving annealing.

Solution to Problem

In order to achieve the above object, the present invention adopts the following configurations.

[1] A steel plate according to an aspect of the present invention includes,
as a chemical composition, by mass%:
- C: 0.07% to 0.11%;
- Si: 0.10% to 0.15%;
- Mn: 0.70% to 1.20%;
- Ni: 1.00% to 2.50%;
- Cr: 0.20% to 0.80%;
- Mo: 0.20% to 0.80%;
- V: 0.005% to 0.070%;
- Al: 0.010% to 0.100%;
- B: 0.0005% to 0.0030%;
- N: 0.0015% to 0.0050%;
- P: 0.006% or less;
- S: 0.0030% or less;
- Cu: 0% to 1.00%;
- Nb: 0% to 0.030%;
- Ti: 0% to 0.010%;
- Ca: 0% to 0.0030%;
- Mg: 0% to 0.0030%;
- REM: 0% to 0.0030%;
- O: 0.0040% or less; and
- a remainder: Fe and impurities,
- in which an α value defined by Equation (1) is 1.00 to 1.50 mass%,
- a β value defined by Equation (2) is 10.0 to 15.0,
- a γ value defined by Equation (3) is 0.70 to 1.50 mass%,
- a Ceq value defined by Equation (4) is 0.550 to 0.620 mass%,
- a yield strength is 670 to 870 N/mm²,
- a tensile strength is 780 to 940 N/mm²,
- a Charpy absorbed energy at -65°C is 100 J or more,
- in measurement of a hardness distribution at a pitch of 0.05 mm in 1 mm × 1 mm at a 1/4 thickness position, an average value of hardness at 121 measurement positions is 265 Hv to 290 Hv and a standard deviation is 20 or less, and
- a plate thickness is 10 to 60 mm. $α = [C] + 6 \times [Si] + 100 \times [P]$ β = 0.65 × [C]^1/2 × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) $γ = [Mn] + 20 \times [Nb] + 36 \times [Ti]$ $Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5$
In Equations (1) to (4), [C] is a C content, [Si] is an Si content, [P] is a P content, [Mn] is an Mn content, [Cu] is a Cu content, [Ni] is an Ni content, [Cr] is a Cr content, [Mo] is an Mo content, [Nb] is an Nb content, [Ti] is a Ti content, and [V] is a V content (mass%), and for amounts of elements that are not contained, including amounts of elements incorporated as impurities, 0 is substituted.
[2] In the steel plate according to [1], [fB] obtained by Equations (A) to (E) may be 0.0003 mass% or more. $[fB] = [B] - 0.77 \times [fN]$ $[fN] = [N] - 0.29 \times [fTi] - 0.52 \times [fAl]$ $[fTi] = [Ti] - 2 \times [fO]$ $[fAl] = [Al] - 1.125 \times [fO]$ $[fO] = [O] - 0.4 \times [Ca] - 0.66 \times [Mg] - 0.11 \times [REM]$ In Equations (A) to (E), [B] is a B content, [N] is an N content, [Ti] is a Ti content, [Al] is an Al content, [O] is an O content, [Ca] is a Ca content, [Mg] is an Mg content, and [REM] is an REM content (mass%), for amounts of elements that are not contained, including amounts of elements incorporated as impurities, 0 is substituted, and in a case where calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
[3] In the steel plate according to [1] or [2], in a case where regions surrounded by grain boundaries having a crystal orientation difference of 15° or more, determined by performing crystal orientation analysis using an electron backscatter diffraction method, are defined as crystal grains, circle equivalent grain sizes of the crystal grains are defined as grain sizes and a value calculated by area-weighted averaging in which weighting is performed by an area for each crystal grain is defined as an average grain size, the average grain size at a 1/4 thickness position may be 15.0 µm or less.
[4] In the steel plate according to any one of [1] to [3], in a case where the steel plate is subjected to stress relieving annealing at a holding temperature of 600°C, a holding time of 2 hours, and a temperature rising rate and a temperature dropping rate of 55 °C/hr or slower in a temperature range of 425°C or higher, at a portion subjected to the stress relieving annealing, a yield strength may be 670 to 870 N/mm², a tensile strength may be 780 to 940 N/mm², and a Charpy absorbed energy at -40°C may be 27 J or more.

Advantageous Effects of Invention

According to the aspect of the present invention, it is possible to provide a steel plate that has excellent strength of a base plate and low-temperature toughness of the base plate and a welded heat-affected zone and also has excellent strength of the base plate and low-temperature toughness of the base plate and the welded heat-affected zone after stress relieving annealing. The steel plate is suitable for a liquefied CO₂ transportation tank.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a steel plate according to an embodiment of the present invention (steel plate according to the present embodiment) will be described in detail.
In the present embodiment, the term "stress relieving annealing" means stress relieving annealing conforming to the content specified in JIS Z 3700: 2022 "methods of post-weld heat treatment", unless otherwise specified. In the present embodiment, the term "welding" means welding in which welding heat input is 1.1 to 4.5 kJ/mm, unless otherwise specified. These conditions are general conditions in the technical field to which the present invention belongs. However, even in a case where stress relieving annealing or welding is performed under conditions different from the above-described conditions, the same effects as those of the stress relieving annealing or welding performed under the above-described conditions can be obtained. Therefore, the steel plate according to the present embodiment may be subjected to stress relieving annealing or welding under conditions different from the above-described conditions.
First, the content of each element constituting the chemical composition of the steel plate according to the present embodiment and reasons for limiting the content will be described. Hereinafter, unless otherwise specified, "%" relating to the content of an element indicates mass%.

(C: 0.07% to 0.11%)

C is an element that improves the strength of a base plate. In order for the steel plate according to the present embodiment to obtain a target strength, the C content is set to 0.07% or more. The C content is preferably 0.08% or more.
Meanwhile, in a case where C is contained in a large amount, the hardness of a welded heat-affected zone increases simultaneously with a decrease in toughness of the welded heat-affected zone. Therefore, the C content is set to 0.11% or less. The C content is preferably set to 0.10% or less, and more preferably set to less than 0.10%.

(Si: 0.10% to 0.15%)

Si is an element that is generally contained in a steel as a deoxidizing element in many cases. The Si content is set to 0.10% or more to contain Si for deoxidation.
Meanwhile, Si is an element that reduces the toughness of a steel after stress relieving annealing. In addition, the Si content is preferably low to suppress a decrease in toughness of a welded heat-affected zone after stress relieving annealing (SR). Therefore, in the steel plate according to the present embodiment, the Si content is set to 0.15% or less. The Si content is preferably set to 0.14% or less, more preferably 0.13% or less, and still more preferably 0.12% or less.

(Mn: 0.70% to 1.20%)

Mn is an effective element for deoxidation and improves the strength of a steel. Therefore, the Mn content is set to 0.70% or more. The Mn content is preferably set to 0.90% or more.
Meanwhile, in a case where Mn is excessively contained, there is a concern that the toughness of a steel after stress relieving annealing may be impaired due to tempering embrittlement. Therefore, the Mn content is set to 1.20% or less. The Mn content is preferably set to 1.10% or less.

(Ni: 1.00% to 2.50%)

Ni is an effective element for improving the hardenability and toughness of a steel. Therefore, the Ni content is set to 1.00% or more. The Ni content is preferably set to 1.20% or more.
Meanwhile, in a case where Ni is excessively contained, there is a concern that the toughness of a steel may decrease after stress relieving annealing. In addition, there is a concern that the toughness of a welded heat-affected zone may deteriorate after stress relieving annealing. Therefore, the Ni content is set to 2.50% or less. The Ni content is preferably set to 2.00% or less.

(Cr: 0.20% to 0.80%)

Cr is an effective element for improving the hardenability of a steel and improving the strength of the steel by precipitation hardening during tempering. Therefore, the Cr content is set to 0.20% or more. The Cr content is preferably set to 0.40% or more.
Meanwhile, in a case where Cr is excessively contained, there is a concern that the toughness of a base plate and a welded heat-affected zone may decrease after stress relieving annealing. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably set to 0.70% or less.

(Mo: 0.20% to 0.80%)

Similar to Cr, Mo is an effective element for improving hardenability and improving the strength of a steel by precipitation hardening during tempering. Therefore, the Mo content is set to 0.20% or more. The Mo content is preferably set to 0.30% or more, more preferably 0.35% or more, and still more preferably 0.40% or more.
Meanwhile, in a case where Mo is excessively contained, there is a concern that Mo carbides precipitate at grain boundaries after stress relieving annealing and the toughness of a base plate and a welded heat-affected zone may decrease, and particularly that the welded heat-affected zone is significantly affected. Therefore, the Mo content is set to 0.80% or less. The Mo content is preferably set to 0.60% or less.

(V: 0.005% to 0.070%)

Similar to Cr and Mo, V is an effective element for improving hardenability and improving the strength of a steel by precipitation hardening during tempering. Therefore, the V content is set to 0.005% or more. The V content is preferably set to 0.010% or more.
Meanwhile, in a case where V is excessively contained, there is a concern that the toughness of a base plate and a welded heat-affected zone may decrease after stress relieving annealing. Therefore, the V content is set to 0.070% or less. The V content is preferably set to 0.050% or less.

(Al: 0.010% to 0.100%)

Al is a useful element for deoxidation and refines the grain size during quenching by forming a nitride. Al is also an essential element for securing [fB] by forming a nitride. Therefore, in the steel plate according to the present embodiment, the Al content is set to 0.010% or more. In a case where the N content is high, the Al content is preferably 0.030% or more, and more preferably 0.040% or more to fix N and secure fB.
Meanwhile, in a case where Al is excessively contained, there is a concern that Al may form a coarse nitride and decrease the toughness of a base plate and a welded heat-affected zone. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.080% or less.

(B: 0.0005% to 0.0030%)

B is an element that improves the hardenability of a steel by being contained in a small amount in the steel plate according to the present embodiment. Therefore, the B content is set to 0.0005% or more. The B content may be set to 0.0006% or more, 0.0008% or more, or 0.0010% or more.
Meanwhile, in a case where B is excessively contained, B may form a coarse nitride and/or carbide and decrease the toughness of a base plate. Therefore, the B content is set to 0.0030% or less. The B content may be set to 0.0020% or less or 0.0010% or less.

(N: 0.0015% to 0.0050%)

N is an element that forms a nitride, refines the grain size of a base plate, and improves toughness. Therefore, the N content is set to 0.0015% or more. The N content may be set to 0.0030% or more or 0.0035% or more.
Meanwhile, in a case where N is excessively contained, nitrides coarsen and the toughness of a welded heat-affected zone as-welded (as weld) decreases. Therefore, the N content is set to 0.0050% or less.

(P: 0.006% or less)

(S: 0.0030% or less)

P and S are impurity elements contained in a steel, and the P content and the S content are preferably as small as possible. Therefore, the lower limits of the P content and the S content are 0%. In the steel plate according to the present embodiment, the P content is set to 0.006% or less, and the S content is set to 0.0030% or less to improve the toughness of a base plate and the toughness of the base plate and the welded part after stress relieving annealing. The P content is preferably set to 0.005% or less. The S content may be set to 0.0020% or less.

(Cu: 0% to 1.00%)

Since Cu is not an essential element in the steel plate according to the present embodiment, the lower limit of the Cu content is 0%. However, since Cu has an effect of improving the strength of a steel, Cu can be contained as necessary. In a case where Cu is contained, the Cu content is preferably 0.10% or more, and more preferably 0.20% or more to utilize the above effect. The Cu content may be set to 0.25% or more or 0.30% or more as necessary.
Meanwhile, in a case where Cu is excessively contained, there is a concern that cracking may occur on a surface of the steel plate and the toughness of a base plate may decrease due to the precipitation of Cu. Therefore, the Cu content is set to 1.00% or less. The Cu content is preferably set to 0.80% or less. The Cu content may be set to 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less as necessary.

(Nb: 0% to 0.030%)

Since Nb is not an essential element in the steel plate according to the present embodiment, the lower limit of the Nb content is 0%. However, since Nb is an element that refines crystal grains during quenching, Nb can be contained as necessary. In a case where Nb is contained, the Nb content is preferably 0.001 % or more to utilize the above effect.
Meanwhile, in a case where Nb is excessively contained, there is a concern that Nb may form a coarse carbonitride and decrease the toughness of a base plate. Therefore, the Nb content is set to 0.030% or less. Since the toughness of a welded heat-affected zone is improved as the amount of Nb is reduced, the Nb content may be set to 0.020% or less, 0.010% or less, or 0.005% or less.

(Ti: 0% to 0.010%)

Since Ti is not an essential element in the steel plate according to the present embodiment, the lower limit of the Ti content is 0%. However, since Ti may refine crystal grains when the steel reaches a high temperature by slab heating or the like, Ti can be contained as necessary. In a case where Ti is contained, the Ti content is preferably set to 0.001% or more to utilize the above effect.
Meanwhile, similar to Nb, in a case where Ti is excessively contained, there is a concern that Ti may form a coarse carbonitride and decrease the toughness of a base plate. Therefore, the Ti content is set to 0.010% or less. The Ti content may be set to 0.005% or less or 0.002% or less as necessary.

(Ca: 0% to 0.0030%)

(Mg: 0% to 0.0030%)

(REM: 0% to 0.0030%)

The steel plate according to the present embodiment may contain one or more of Ca, Mg, and REM. Since Ca, Mg, and REM are not essential elements, the lower limits of the Ca content, the Mg content, and the REM content are all 0%.
Ca has an effect of reducing the influence of MnS, which decreases the toughness of the steel plate, by making a sulfide in the steel plate spherical. The Ca content may be set to 0.0001% or more to obtain the above effect.
Meanwhile, in a case where Ca is contained in a large amount, there is a concern that the weldability of a steel may be impaired. Therefore, the Ca content is set to 0.0030% or less. The Ca content may be set to 0.0015% or less, 0.0010% or less, 0.0005% or less, or 0.0002% or less as necessary.
Mg and REM improve the toughness of a welded heat-affected zone by forming an oxide. Each of the Mg content and the REM content may be set to 0.0001% or more to obtain the above effect.
Meanwhile, in a case where Mg and REM are contained in large amounts, there is a concern that a coarse oxide may be formed and the toughness of a steel may decrease. Therefore, each of the Mg content and the REM content is set to 0.0030% or less. Each of the Mg content and the REM content may be set to 0.015% or less, 0.010% or less, 0.005% or less, or 0.002% or less, or less than 0.0015% as necessary. REM is a general term for rare earth metals, including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. They have stronger deoxidizing properties than other additional elements, and form a stable oxide in the steel.

(O: 0.0040% or less)

Oxygen (O) is an impurity element contained in a steel. In the steel, oxygen forms an oxide having a size of several µm to several tens of µm together with Ca, Mg, REM, Al, Ti, and the like having a strong deoxidizing force in many cases. In a case where a coarse oxide is contained or the oxide number density is high, the oxide may serve as an origin of brittle fracture. Therefore, the O content is preferably as small as possible. Therefore, the lower limit of the O content is 0%. In the steel plate according to the present embodiment, the O content is set to 0.0040% or less, and preferably 0.0030% or less to improve the toughness of the welded part.

(Remainder: Fe and impurities)

The steel plate according to the present embodiment contains a remainder consisting of Fe and impurities, in addition to the above-described components. Here, the impurities refer to elements that are, when a steel is industrially produced, incorporated from raw materials such as ore or scrap or incorporated by various factors in the production process, and the impurities are allowed to be included in the steel in a range not adversely affecting the present invention.
Furthermore, in the steel plate according to the present embodiment, [fB] obtained by Equations (A) to (E) is preferably 0.0003 mass% or more. [fB] represents the amount of B solid-solubilized in a steel. When [fB] is set to 0.0003 mass% or more, it is possible to increase the hardenability of a high-tensile-strength steel having a yield strength of 670 to 870 N/mm² and a tensile strength of 780 to 940 N/mm². B is likely to form a nitride in a steel. In addition, Ti and Al are likely to form a nitride and an oxide. Therefore, the amount of B [fB] solid-solubilized in a steel is obtained by Equations (A) to (E). [fB] may be 0.0005 mass% or more or 0.0015 mass% or more. In addition, [fB] may be 0.0025 mass% or less or 0.0018 mass% or less. $[fB] = [B] - 0.77 \times [fN]$ $[fN] = [N] - 0.29 \times [fTi] - 0.52 \times [fAl]$ $[fTi] = [Ti] - 2 \times [fO]$ $[fAl] = [Al] - 1.125 \times [fO]$ $[fO] = [O] - 0.4 \times [Ca] - 0.66 \times [Mg] - 0.11 \times [REM]$
In Equations (A) to (E), [B] is a B content, [N] is an N content, [Ti] is a Ti content, [Al] is an Al content, [O] is an O content, [Ca] is a Ca content, [Mg] is an Mg content, and [REM] is an REM content (mass%) in the steel plate, and for the amounts of elements that are not contained, including the amounts of elements incorporated as impurities, 0 is substituted. In addition, in a case where calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
In addition, in the steel plate according to the present embodiment, in addition to the limitation of the content of each element, the ranges of an α value, a β value, and a γ value calculated from the contents of the elements are limited as follows.

(α Value: 1.00 to 1.50 mass%)

The α value is represented by Equation (1). $α = [C] + 6 \times [Si] + 100 \times [P]$
[C] is a C content, [Si] is an Si content, and [P] is a P content (mass%) in the steel plate.
In the present embodiment, the α value is set to 1.50 mass% or less. This is a condition required to improve the toughness of a part where the grains are coarsened in the base plate or the welded heat-affected zone after stress relieving annealing, and C, Si, and P are required to be adjusted within a range in which the condition is satisfied. After the SR treatment, the boundary segregation concentration of P increases, and thus brittle fracture is likely to occur at grain boundaries. However, brittle fracture can be controlled by P, C, and Si. P is necessarily contained in the steel in consideration of the production process. P is a representative element that causes SR embrittlement since it significantly reduces the grain boundary strength by boundary segregation, and its coefficient is the highest. C and Si are also elements that are necessarily contained in the steel. An increase in the amounts of these elements causes embrittlement due to cementite generated at grain boundaries. It is desirable to reduce the amounts of these elements, but they may be contained in certain amounts due to characteristics or in compliance with a standard. In order to improve the toughness after SR, the α value is preferably set to 1.40 mass% or less. The α value is 1.00 mass% or more. The above lower limit is a range determined by component restrictions in the standard in the field of application, limitations in element control in the production, and the like, and is calculated by substituting the lower limits of the C content, Si content, and P content described above and their realistic minimum values in the production into Equation (1). A preferable lower limit of the α value can be calculated from preferable lower limits of the C content, Si content, and P content. The α value may be more than 1.10 mass% or 1.30 mass% or more.

(β Value: 10.0 to 15.0)

The β value is calculated by Equation (2). β = 0.65 × [C]^1/2 × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo])
[C] is a C content, [Si] is an Si content, [Mn] is an Mn content, [Cu] is a Cu content, [Ni] is an Ni content, [Cr] is a Cr content, and [Mo] is an Mo content (mass%) in the steel.
In the steel plate according to the present embodiment, the range of the β value is set to 10.0 to 15.0. The β value is an index indicating the hardenability of the steel, and the higher the β value, the more likely it is to stably avoid the formation of an upper bainite structure inferior in terms of a balance between strength and toughness. However, when the β value is too high, the strength of the steel increases, and thus the toughness deteriorates. That is, the β value is also an index indicating a target range of the content of the alloying element necessary for improving the toughness of a welded heat-affected zone as-welded. The β value may be set to 11.0 or more as necessary. At the same time, the β value may be set to 14.0 or less.

(γ Value: 0.70 to 1.50 mass%)

The γ value is calculated by Equation (3). $γ = [Mn] + 20 \times [Nb] + 36 \times [Ti]$
[Mn] is an Mn content, [Nb] is an Nb content, and [Ti] is a Ti content (mass%) in the steel plate.
In the steel plate according to the present embodiment, the range of the γ value is set to 0.70 to 1.50 mass%. Mn, Nb, and Ti are all elements that promote intergranular embrittlement after stress relieving annealing. By setting the γ value to 1.50 mass% or less, it is possible to suppress a decrease in toughness of a base plate or a welded heat-affected zone after stress relieving annealing. Several mechanisms by which these elements promote intergranular embrittlement are considered, and embrittlement due to a decrease in grain boundary strength caused by boundary segregation or carbonitrides generated at grain boundaries is considered. The γ value may be 1.40 mass% or less.
Meanwhile, in order to ensure certain hardenability and obtain a microstructure superior in terms of a balance between strength and toughness, certain amounts of Mn, Nb, and Ti are preferably added, and the γ value is set to 0.70 mass% or more. The γ value may be 0.75 mass% or more.
In a case where the numerical value ranges relating to the α value, the β value, and the γ value are satisfied, the range of fields in which a steel having excellent low-temperature toughness in a welded part (welded heat-affected zone) even as-welded and after stress relieving annealing can be provided expands.
By controlling the α value and the β value within certain ranges, it is possible to produce a steel having excellent low-temperature toughness in a welded part even as-welded and after stress relieving annealing, but in some cases, depending on the intended use, the α value may not be freely set due to the chemical composition regulations of the application standard, and the α value has to be increased. In contrast, in the steel plate according to the present embodiment, the γ value is newly set, and thus it is possible to expand the allowable range of the α value. Accordingly, even in a field where the chemical composition regulations of the application standard are strict as described above, it is possible to provide a steel having excellent low-temperature toughness in a welded part even as-welded and after stress relieving annealing.
In addition, in the steel plate according to the present embodiment, a carbon equivalent Ceq, which is calculated by Equation (4) and is an index indicating the hardenability of the steel, is set to 0.550 to 0.620 mass%. $Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5$
In Equation (4), [C] is a C content, [Mn] is an Mn content, [Cu] is a Cu content, [Ni] is an Ni content, [Cr] is a Cr content, [Mo] is an Mo content, and [V] is a V content (mass%) in the steel plate.
In a case where Ceq is less than 0.550 mass%, the steel plate may have insufficient strength. Ceq may be set to 0.570 mass% or more or 0.600 mass% or more as necessary. In addition, in a case where Ceq is more than 0.620 mass%, the steel plate may have decreased toughness. Ceq may be set to 0.600 mass% or less as necessary.

(Yield Strength: 670 to 870 N/mm²)

(Tensile Strength: 780 to 940 N/mm²)

In the steel plate according to the present embodiment, the yield strength is set to 670 to 870 N/mm², and the tensile strength of the steel plate is set to 780 to 940 N/mm². In order to reduce the weight of a large-sized welded structure such as a transportation tank for liquefied CO₂, a steel plate is required that can ensure the strength of the structure even when the plate thickness is small. Usually, a steel plate selected for such an intended use has the above-described yield strength and tensile strength. Therefore, in the steel plate according to the present embodiment, the yield strength and the tensile strength are also set to the above-described ranges. The yield strength may be set to 690 N/mm² to 830 N/mm² as necessary. In addition, the tensile strength may be set to 800 N/mm² to 900 N/mm².

(Charpy Absorbed Energy at -65°C is 100 J or More)

In addition, the steel plate according to the present embodiment is required to have a Charpy absorbed energy at -65°C of 100 J or more to ensure high toughness. Therefore, it is possible to ensure the safety of a transportation tank formed of the steel plate according to the present embodiment. The Charpy absorbed energy at -65°C is a numerical value measured at a position (may be referred to as a t/4 position or 1/4 thickness position) 1/4 of the plate thickness away from the surface in the plate thickness direction.

(δ Value of CTOD Test at -35°C is 0.10 mm or More)

Recently, the fracture resistance characteristics of a welded structure have been evaluated using a fracture mechanical evaluation method and incorporated into the design to ensure the safety of a welded structure such as a transportation tank. Specifically, as brittle fracture occurrence characteristics, a crack opening displacement amount (hereinafter, abbreviated as δc) called a CTOD value is obtained as a fracture mechanical parameter by a crack tip opening displacement test (CTOD test) specified by the Japan Welding Engineering Society Standards WES1108 or the like, and whether δc can satisfy the design criterion is evaluated in many cases.
In order to improve δc of a material, it is necessary to enhance the characteristics of the material from a viewpoint different from that in the related art. In the related art, a Charpy impact test has been used as a method of evaluating the brittle fracture resistance of a material. The value obtained from the Charpy impact test represents the average toughness of an evaluation target region. However, in the CTOD test, even in a case where the average toughness is good in an evaluation target region, when there is a part where the toughness is even slightly weak in the evaluation target region, the presence thereof is reflected in δc. δc has such properties. Therefore, particularly in a region where the microstructure of a steel changes in a non-uniform and complicated manner, such as a welded heat-affected zone, it is necessary to reduce the size of the local embrittled region as much as possible in order to obtain a high δc value.
In the steel plate according to the present embodiment, a δ value of the CTOD test at -35°C is preferably 0.10 mm or more to ensure high toughness. In this case, the safety of a transportation tank formed of the steel plate according to the present embodiment is further improved.

(Average Value and Standard Deviation of Hardness)

The steel plate according to the present embodiment is required to have, in the measurement of a hardness distribution at a pitch of 0.05 mm in a range of 0.5 mm × 0.5 mm at a 1/4 thickness position, an average value of hardness of 265 Hv to 290 Hv at 121 measurement positions and a standard deviation of 20 or less. The structure of the steel plate according to the present embodiment is preferably a mixed structure of a martensite structure and a lower bainite structure superior in terms of a balance between strength and toughness. However, due to an unevenness in local γ grain size or microsegregation, a partial decrease in hardenability may occur and an upper martensite structure inferior in terms of a balance between strength and toughness may thus be formed. In a case where there is an upper bainite structure, the hardness distribution becomes non-uniform, and there is a concern that the toughness of a base plate may deteriorate. In a case where the average value of hardness is less than 265 Hv or the standard deviation is more than 20, the upper bainite structure may be included, and the toughness of the base plate cannot be ensured. Meanwhile, in a case where the average value is more than 290 Hv, there is a concern that the toughness may decrease due to an excessive increase in strength.
For the measurement of the hardness distribution described above, a micro-sample is collected so that a surface (L-cross section) parallel to the rolling direction of the steel plate and the plate thickness direction is provided as an observation surface, and the measurement is performed using a micro-Vickers hardness tester. As a measurement region, a range of 0.5 mm × 0.5 mm whose center is at any t/4-position in the micro-observation surface is set, and at a total of 121 points of 11 points in the longitudinal direction × 11 points in the lateral direction, the measurement is performed at a measurement pitch of 0.05 mm with a measurement load of 25 gf. The average value and the standard deviation are calculated from the obtained measured values.

(Plate Thickness: 10 to 60 mm)

In general, in a case where a steel plate having a plate thickness of less than 10 mm is welded, stress relieving annealing (SR) is not required. However, since a steel plate requiring SR is targeted as the steel plate according to the present embodiment, the plate thickness is set to 10 mm or more. The plate thickness is preferably 25 mm or more. Meanwhile, a steel plate having a plate thickness of more than 60 mm is not preferable since its contribution to the reduction in weight of a transportation tank to which the steel plate is applied is small. Therefore, the plate thickness of the steel plate according to the present embodiment is set to 60 mm or less.
Furthermore, the steel plate according to the present embodiment may have a configuration to be described below.

(Structure)

In the steel plate according to the present embodiment, a structure at a 1/4 thickness position in a cross section in the plate thickness direction is preferably a mixed structure of a martensite structure and a lower bainite structure in order to satisfy the average value and the standard deviation of hardness at 121 points described above. The martensite structure and the lower bainite structure may account for 85 area% or more in total.

(Average Grain Size at 1/4 Thickness Position in Steel Plate: 15.0 µm or Less)

In the steel plate according to the present embodiment, the average grain size at a 1/4 thickness position may be set to 15.0 µm or less. The average grain size may be set to 14.5 µm or less or 14.0 µm or less as necessary to improve the toughness of a base plate and the toughness of the base plate after SR. Since the average grain size at the 1/4 thickness position in the steel plate is preferably as small as possible, it is not necessary to specify the lower limit of the average grain size. Regarding the average grain size, the smallest average grain size is usually about 10.0 µm.
The average grain size is defined as follows.
A sample in which an L-cross section of the steel plate can be observed is prepared and a 1/4 thickness position in the L-cross section is set as an observation part to perform crystal orientation analysis using an electron backscatter diffraction method (EBSD method) using a scanning electron microscope at a pitch of 0.5 µm in a range of 200 µm in the plate thickness direction and 250 µm in the rolling direction. From the results of the crystal orientation analysis, regions surrounded by grain boundaries having a crystal orientation difference of 15° or more are defined as crystal grains, circle equivalent grain sizes of the crystal grains are defined as grain sizes, and a value calculated by area-weighted averaging in which weighting is performed by the area for each crystal grain is set as an average grain size.

(Charpy Absorbed Energy at -40°C After Stress Relieving Annealing is 27 J or More)

In order to prevent fracture in advance, the steel plate according to the present embodiment is subjected to stress relieving annealing on a welded part after being assembled in a transportation tank. However, in this case, not only the welded part but also the base plate are heated. When the base plate is heated, the toughness of the base plate tends to decrease. The reason is not clear, but it is presumed that phosphorus (P) diffuses to grain boundaries and inclusions grow or aggregate in the structure, so that the brittleness decreases and the toughness thus decreases. Therefore, in the steel plate according to the present embodiment, the Charpy absorbed energy at -40°C after stress relieving annealing is preferably 27 J or more. In this case, the safety can be further increased.
The Charpy absorbed energy at -40°C after stress relieving annealing is measured at a portion subjected to the stress relieving annealing in a case where the steel plate is subjected to the stress relieving annealing at a holding temperature of 600°C, a holding time of 2 hours, and a temperature rising rate and a temperature dropping rate of 55 °C/hr or slower in a temperature range of 425°C or higher.

(Yield Strength of 670 to 870 N/mm² and Tensile Strength of 780 to 940 N/mm² After Stress Relieving Annealing)

The steel plate according to the present embodiment preferably has a yield strength of 670 to 870 N/mm² and a tensile strength of 780 to 940 N/mm² after stress relieving annealing. Therefore, sufficient strength can be ensured for a transportation tank for liquefied CO₂ subjected to stress relieving annealing.
In addition, in the steel plate according to the present embodiment, the δ value of the CTOD test at -35°C is preferably 0.10 mm or more even after the stress relieving annealing. In this case, the safety is further improved.
The steel plate according to the present embodiment has excellent toughness (as-welded, after stress relieving annealing) in a welded heat-affected zone since it has the above-described configuration.
The toughness of the welded heat-affected zone is not limited. However, in a case where it is as-welded, the Charpy absorbed energy at -65°C is preferably 70 J or more, and after stress relieving annealing, the Charpy absorbed energy at -65°C is preferably 70 J or more as a target value.
In addition, in the welded heat-affected zone, even in a case where it is either as-welded or after stress relieving annealing, the δ value of the CTOD test at -35°C is more preferably 0.10 mm or more.
Next, a method of producing the steel plate according to the present embodiment will be described below.
Effects can be obtained as long as the steel plate according to the present embodiment has the above-described characteristics regardless of a production method. However, since the steel plate can be stably produced in a case where a production method to be described below is used, it is preferable to use the production method.
In order to produce a steel having the above-described chemical composition as a steel plate, a usual method of producing a steel product may be used. That is, for example, a steel produced by a converter method or an electric furnace method and refined by secondary refining equipment is made into a slab by continuous casting or ingot blooming. The slab thickness may be adjusted so that segregation is reduced and the material quality is improved by porosity reduction, and the slab thickness for this purpose is preferably 150 mm or more. The upper limit of the slab thickness is not particularly limited, and the slab thickness may be 600 mm or less or 400 mm or less, for example.
Thereafter, the slab is preferably heated to about 950°C to 1,250°C by a slab heating furnace, and then rolled up to a predetermined plate thickness by hot rolling under conditions to be described later to obtain a steel plate. Furthermore, the steel plate is subjected to quenching and tempering to obtain a steel plate (final steel plate) having predetermined characteristics.
In the steel plate according to the present embodiment, the P content is required to be reduced to 0.006% or less. In a usual dephosphorization method, the P content cannot be reduced to 0.006% or less in some cases. In this case, measures such as extending a dephosphorization treatment time may be adopted.
In the hot rolling, it is desirable that the cumulative rolling reduction at a rolling temperature of 1,150°C to 900°C be set to 50% or more. There is no need to particularly specify the upper limit of the cumulative rolling reduction in the above temperature range, and the cumulative rolling reduction may be 80% or less or 70% or less.
Regarding quenching and tempering, in a case where the plate thickness is 50 mm or less or less than 50 mm, a direct quenching treatment including immediate water cooling after hot rolling may be performed and a reheating and quenching treatment to be described later may be omitted. In a case where the direct quenching treatment is performed, the cooling start temperature is set to an Ar3 point or higher and water cooling is performed up to 300°C or lower. The average cooling rate during water cooling is preferably set to 5 °C/sec or faster in a range of 700°C to 300°C in the temperature history during cooling of the front and rear surfaces of the steel plate. The upper limit of the average cooling rate is not particularly limited, and the average cooling rate may be, for example, 100 °C/sec or slower, 50 °C/sec or slower, or 20 °C/sec or slower. After direct quenching, reheating and further quenching may be performed.
The Ar3 point is obtained using the following equation. Ar3 = 910 - 310 × [C] - 8 × [Mn] - 20 × [Cu] - 15 × [Cr] - 55 × [Ni] - 80 × [Mo] + 0.35 × (t - 8)
Here, in the equation, [C] is a C content, [Mn] is an Mn content, [Cu] is a Cu content, [Cr] is a Cr content, [Ni] is an Ni content, and [Mo] is an Mo content by mass% in the steel plate, and t is a plate thickness of the steel plate by mm.
In a case where the plate thickness is 50 mm or more, the quenching treatment is preferably performed by cooling the steel plate once after rolling and then reheating the steel plate. In a case where the plate thickness is 50 mm or more, when a reheating and quenching treatment is performed, direct quenching after hot rolling may be omitted, or direct quenching may be performed.
In a case where reheating is performed, the heating temperature (that is, quenching temperature) during the quenching treatment is desirably set to 925°C or lower, and may be 920°C or lower, 915°C or lower, or 910°C or lower. This because, in a case where the steel plate is thick, the metallographic structure may not be sufficiently refined after rolling. In a case where the quenching temperature for a steel plate in which the metallographic structure is not sufficiently refined is higher than 925°C, the reverse-transformed γ structure formed with heating coarsens, and the average grain size of the final structure after γ/α transformation by subsequent cooling also coarsens.
Meanwhile, it is not preferable that the lower limit of the quenching temperature be a temperature (for example, a temperature of an Ac3 point or higher and Ac3 point + 20°C or lower) slightly above the Ac3 point, since the hardenability may be insufficient due to an unevenness in reverse transformation γ grain size or insufficient solid-solubilization of carbides containing B. Therefore, the quenching temperature is preferably 880°C or higher, and more preferably 890°C or higher. In the description of the quenching treatment conditions, it is assumed that the plate thickness of the steel plate is 50 mm or more. However, the quenching treatment conditions are also applied to a case where a steel plate having a plate thickness of less than 50 mm is reheated and quenched.
In the present embodiment, tempering is performed after quenching (after direct quenching or reheating and quenching, or after reheating and quenching in a case where both of the direct quenching and the reheating and quenching are performed). The heating temperature (that is, tempering temperature) during tempering is desirably set to 660°C or lower. In a case where the tempering temperature is higher than 660°C, the tempering effect may be excessively exhibited. Therefore, it may be difficult to ensure the yield stress and the tensile strength or the toughness may decrease. The tempering temperature is set to 500°C or higher, and preferably 600°C or higher. In a case where the tempering temperature is too low, tempering is insufficient. Therefore, it is difficult to ensure a predetermined yield stress and a predetermined tensile strength.
In a case where cooling is performed after reheating and quenching or after tempering, it is desirable to cool the steel plate (perform accelerated cooling) by water cooling instead of air cooling in order to prevent a decrease in toughness of the base plate due to tempering embrittlement. In this case, the average cooling rate up to 300°C is preferably set to 0.1 °C/sec or faster or 0.5 °C/sec or faster.
The steel plate according to the present embodiment is suitable as a steel plate for a liquefied CO₂ transportation tank. For example, the steel plate can be used for a transportation tank to be mounted on ship. In CO₂ transportation by ship, liquefied CO₂ is filled in a transportation tank provided in the ship and transported. However, in order to prevent CO₂ from being solidified (converted into dry ice) in the transportation tank, it is preferable to transport CO₂ while holding it at a pressure of about 2 MPa. In addition, it is preferable to keep CO₂ at about -35°C in order to maintain CO₂ in a liquid state at a pressure of about 2 MPa. The steel plate according to the present embodiment can be suitably used for such an intended use.

Examples

Next, examples of the present invention will be described. Conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, but the present invention is not limited to this example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Hot metal subjected to a blast furnace treatment was tapped off into a hot-metal ladle, and then subjected to a hot metal pretreatment such as desulfurization. Then, the hot metal was inserted into a converter. Next, a dephosphorization treatment was performed in the converter, and the P content was adjusted to 0.006% or less.
The molten steel subjected to the dephosphorization treatment was further subjected to component adjustment. Then, slabs having chemical compositions shown in Tables 1A and 1B were cast.
Thereafter, the slabs were heated to heating temperatures shown in the table by a heating furnace, and then rolled up to predetermined plate thicknesses by hot rolling to obtain steel plates.
Furthermore, the steel plates were subjected to quenching and tempering to obtain steel plates (final steel plates) having predetermined characteristics. Table 2 shows heating temperatures before rolling, cumulative rolling reductions at 1,150°C to 900°C in hot rolling, plate thicknesses after rolling, quenching temperatures, and tempering temperatures. Cooling after reheating and quenching or after tempering was performed by water cooling and the average cooling rate up to 300°C was set to 0.1 °C/sec or faster. Some steel plates were directly subjected to a direct quenching treatment in which the steel plates were immediately water-cooled after hot rolling. Cooling start temperatures, cooling end temperatures, and average cooling rates in this case are shown in the table.
Table 1A and Table 1B show chemical compositions, α values, β values, γ values, fB values, and carbon equivalents Ceq of the steel plates. In addition, in the column of before SR in characteristics of base plate in Table 3A, average values (average Hv) of base plate hardness at 121 measurement positions, average grain sizes (EBSD grain size), yield strengths (MPa), tensile strengths (MPa), yield ratios, Charpy absorbed energies (J) at -65°C, and δ values (mm) of a CTOD test at -35°C are shown.
For the measurement of the EBSD grain size, a sample in which an L-cross section of the steel plate could be observed was prepared and a 1/4 thickness position in the L-cross section was set as an observation part to perform crystal orientation analysis using an electron backscatter diffraction method (EBSD method) using a scanning electron microscope at a pitch of 0.5 µm in a range of 200 µm in the plate thickness direction and 250 µm in the rolling direction. From the results of the crystal orientation analysis, regions surrounded by grain boundaries having a crystal orientation difference of 15° or more were defined as crystal grains, circle equivalent grain sizes of the crystal grains were defined as grain sizes, and a value calculated by area-weighted averaging in which weighting was performed by the area for each crystal grain was set as an average grain size.
A tensile test was performed in accordance with JIS Z 2241: 2011 with a JIS No. 4 round bar test piece having a parallel part of φ14 mm, collected from the 1/4 thickness position in a C direction. Each of the yield strength and the tensile strength was an average value of two test pieces. As the yield strength, a 0.2% proof stress was applied. As the yield ratio, a ratio of a yield strength YS to a tensile strength TS was applied, and the yield ratio is represented by a percentage, that is, 100 × (YS/TS). The unit of the yield ratio is %.
For the measurement of the hardness distribution, a micro-sample was collected so that an L-cross section parallel to the rolling direction of the steel plate and the plate thickness direction was provided as an observation surface, and the observation surface was wet-polished. Then, a mirror surface finished by buffing using 1.0 µm-diamond particles was measured using a micro-Vickers hardness tester. As a measurement region, a range of 0.5 mm × 0.5 mm whose center was at a 1/4t position in the micro-observation surface was randomly selected, and at a total of 121 points of 11 points in the longitudinal direction × 11 points in the lateral direction, the measurement was performed at a measurement pitch of 0.05 mm with a measurement load of 25 gf. An average value and a standard deviation were calculated from the obtained measured values.
In addition, a semi-automatic welded joint with a weld line parallel to the rolling direction was produced and evaluated. Specifically, a K-bevel was processed and multilayer gas shielded arc welding (GMAW) was performed with an argon gas containing 20% of CO₂ as a shielding gas, a welding wire YM-69F manufactured by Nippon Steel Welding & Engineering Co., Ltd. as a welding wire, a heat input amount of 2.0 kJ/mm, and preheating to 100°C to produce a welded joint.
A microstructure was allowed to appear in a C-cross section at the welded part (as weld) of the welded joint, and then Charpy test pieces were collected from a position centered at a position 6.5 mm below the surface (described as surface collection in the table) and from a position centered at the thickness middle portion (described as t/2 collection in the table) with a front I-side fusion line (FL) as a position of a notch center.
The test pieces were subjected to a Charpy test at -65°C to obtain absorbed energy. The results are shown in the column of As weld in characteristics of joint in Table 3B.
In addition, after a microstructure of the welded part was allowed to appear, CTOD test pieces of full thickness were collected with a front I-side fusion line as a position of each notch center, and subjected to a CTOD test at -35°C to obtain δ values. The results are shown in the column of As weld in characteristics of joint in Table 3B.
Thereafter, stress relieving annealing (SR) was performed on the base plate and the welded part. For the stress relieving annealing, the holding temperature was set to 600°C, the holding time was set to 2 hours, and the temperature rising rate and a temperature dropping rate was set to 55 °C/hr or slower in a temperature range of 425°C or higher.
The yield strength and the tensile strength of the base plate after SR were obtained in the same manner as in before SR.
In addition, a Charpy test was performed at -40°C on test pieces each collected in the C direction at a t/4 position in the base plate after SR, and Charpy absorbed energies were obtained. In addition, a CTOD test was performed at -35°C, and δ values were obtained.
These results are shown in the column of after SR in characteristics of base plate in Table 3A.
Furthermore, after a microstructure was allowed to appear in a C-cross section at the welded part after SR, Charpy test pieces centered at a position 6.5 mm below the surface and a t/2 position were collected with a front I-side fusion line as a position of each notch center, and Charpy absorbed energies at -40°C obtained in the test are shown. In addition, after a microstructure of the welded part after SR was allowed to appear, CTOD test pieces of full thickness were collected with a front I-side fusion line as a position of each notch center, and δ values of the CTOD test at -35°C obtained by the test are shown.
These results are shown in the column of after SR in characteristics of joint in Table 3B.
Regarding the Charpy absorbed energies of the base plate and the welded part, three V-notch test pieces were collected from each of the base plate and the welded part, and a Charpy impact test was performed at a predetermined temperature to measure the absorbed energy. As the V-notch test piece, a full-size test piece described in JIS Z 2242: 2005 was collected from each plate thickness position in the C direction. The Charpy impact test was performed in accordance with JIS Z 2242: 2005.
The δ value (δc) of the CTOD test was measured in accordance with BS 7448 standard (British standard) Part 1 (1991) and BS 7448 standard (British standard) Part 2 (1997).
The base plate was evaluated in the C direction (plate width direction) in which the longitudinal direction of the test piece was perpendicular to the rolling direction.
In the welded joint part, gas shielded arc welding was performed with a heat input amount of 35 kJ/mm on a butt part of the steel plate subjected to K-bevel processing, a process is performed so that a tip end of the fatigue notch of the CTOD test piece of the welded part is present at a plate thickness center portion of the I-side fusion line (FL) of the welded part, and a CTOD test was performed at a predetermined temperature. The welded joint was evaluated only in an L direction (rolling direction). In the evaluation of CTOD of the welded joint, a test piece was collected so that a tip end of the fatigue crack corresponded to a weld bond. At each test temperature, the test was performed three times, and the minimum value of the obtained measurement data was set as a δ value of the CTOD test. The unit of CTOD shown in Tables 3A and 3B is mm.
As shown in Tables 1A to 3B, all of Nos. 1 to 14, which are invention examples, had excellent strength and toughness. In particular, even after the SR treatment, excellent low-temperature toughness was exhibited. In addition, after the SR treatment, the yield strength was 670 to 870 N/mm² and the tensile strength was 780 to 940 N/mm², which were good values.
In Nos. 1 to 14, the Charpy absorbed energy in the welded heat-affected zone of the welded joint exceeded 70 J both before the SR treatment (-65°C) and after the SR treatment (-40°C), and the low-temperature toughness was good.
Meanwhile, as shown in Tables 1A to 3B, in Nos. 15 to 45 and 55, which are comparative examples, the chemical composition (the content of each element, α value, β value, γ value, and Ceq.) of the steel plate was out of the range specified in the present invention, and thus the toughness of at least the base plate or the welded heat-affected zone deteriorated.

In Nos. 46 to 54, the chemical composition satisfied the component range of the present invention, but the production conditions did not satisfy preferable production conditions. Therefore, the toughness deteriorated. That is, at least the Charpy absorbed energy at -65°C at the t/4 position was less than 100 J, and in some examples, other toughness was also inferior.

[Table 1A]

No.	Chemical Composition (mass%, Remainder: Fe and Impurities)
No.	C	Si	Mn	Ni	Cr	Mo	V	Al	B	N	P	S
1	0.084	0.13	0.91	2.00	0.52	0.47	0.020	0.070	0.0018	0.0023	0.005	0.0020
2	0.093	0.14	0.79	2.40	0.43	0.47	0.040	0.059	0.0008	0.0032	0.004	0.0018
3	0.091	0.15	1.13	2.30	0.40	0.31	0.060	0.089	0.0007	0.0031	0.005	0.0008
4	0.090	0.14	0.75	1.50	0.65	0.50	0.040	0.072	0.0013	0.0032	0.005	0.0021
5	0.082	0.13	0.83	2.00	0.50	0.47	0.040	0.035	0.0021	0.0035	0.005	0.0019
6	0.091	0.10	0.80	1.20	0.55	0.50	0.040	0.072	0.0021	0.0040	0.006	0.0015
7	0.090	0.13	1.03	1.60	0.67	0.36	0.040	0.056	0.0018	0.0021	0.004	0.0023
8	0.082	0.10	0.91	2.30	0.54	0.45	0.020	0.081	0.0007	0.0023	0.005	0.0013
9	0.073	0.11	1.05	1.80	0.58	0.42	0.040	0.055	0.0017	0.0035	0.004	0.0009
10	0.085	0.15	1.18	2.00	0.25	0.46	0.040	0.079	0.0013	0.0033	0.004	0.0023
11	0.100	0.14	0.91	1.90	0.56	0.41	0.020	0.067	0.0018	0.0040	0.005	0.0011
12	0.091	0.13	1.02	1.60	0.48	0.50	0.010	0.072	0.0016	0.0025	0.006	0.0010
13	0.092	0.15	0.94	1.50	0.35	0.44	0.050	0.042	0.0018	0.0036	0.005	0.0015
14	0.076	0.13	0.95	1.90	0.70	0.47	0.020	0.078	0.0017	0.0025	0.005	0.0010
15	0.095	0.14	1.03	2.30	0.28	0.42	0.040	0.083	0.0019	0.0033	0.006	0.0016
16	0.110	0.15	1.00	1.20	0.58	0.45	0.040	0.055	0.0010	0.0032	0.006	0.0010
17	0.096	0.15	0.82	2.00	0.40	0.42	0.060	0.041	0.0022	0.0028	0.005	0.0010
18	0.082	0.15	0.86	1.60	0.46	0.33	0.040	0.059	0.0012	0.0031	0.005	0.0022
19	0.094	0.12	1.10	2.40	0.52	0.44	0.060	0.042	0.0014	0.0022	0.006	0.0012
20	0.095	0.13	1.18	1.90	0.62	0.45	0.040	0.048	0.0022	0.0031	0.005	0.0015
21	0.083	0.11	0.90	1.80	0.55	0.46	0.020	0.078	0.0012	0.0027	0.004	0.0016
22	0.084	0.12	1.18	1.90	0.39	0.46	0.020	0.083	0.0006	0.0031	0.005	0.0008
23	0.072	0.11	1.09	1.80	0.34	0.43	0.020	0.051	0.0008	0.0023	0.004	0.0012
24	0.092	0.11	0.68	1.70	0.45	0.44	0.060	0.069	0.0009	0.0030	0.006	0.0015
25	0.120	0.12	0.93	1.80	0.49	0.49	0.020	0.071	0.0010	0.0039	0.005	0.0024
26	0.080	0.17	0.97	2.00	0.53	0.47	0.010	0.067	0.0012	0.0027	0.004	0.0013
27	0.084	0.15	1.35	1.80	0.45	0.40	0.030	0.077	0.0016	0.0021	0.005	0.0013
28	0.091	0.13	0.90	2.60	0.61	0.30	0.010	0.069	0.0013	0.0036	0.005	0.0017
29	0.090	0.12	0.82	2.00	0.85	0.30	0.020	0.088	0.0011	0.0024	0.004	0.0010
30	0.074	0.14	0.78	1.70	0.49	0.88	0.060	0.086	0.0021	0.0037	0.005	0.0013
31	0.092	0.11	1.10	2.20	0.37	0.34	0.120	0.090	0.0011	0.0027	0.006	0.0008
32	0.080	0.13	0.97	2.00	0.44	0.45	0.020	0.120	0.0006	0.0027	0.004	0.0010
33	0.084	0.10	1.03	1.70	0.55	0.50	0.020	0.084	0.0000	0.0036	0.004	0.0018
34	0.072	0.12	0.95	1.60	0.58	0.37	0.010	0.090	0.0035	0.0022	0.005	0.0015
35	0.075	0.15	0.92	2.20	0.44	0.46	0.060	0.077	0.0018	0.0056	0.004	0.0022
36	0.090	0.11	0.88	1.80	0.57	0.33	0.020	0.087	0.0016	0.0037	0.007	0.0014
37	0.096	0.11	0.80	2.20	0.34	0.52	0.030	0.084	0.0020	0.0027	0.005	0.0035
38	0.098	0.13	0.79	1.50	0.44	0.49	0.040	0.075	0.0006	0.0026	0.005	0.0013
39	0.076	0.14	0.75	2.20	0.61	0.43	0.030	0.084	0.0010	0.0029	0.005	0.0012
40	0.075	0.13	0.75	2.20	0.67	0.37	0.050	0.071	0.0015	0.0032	0.005	0.0013
41	0.092	0.11	0.75	2.00	0.60	0.45	0.060	0.071	0.0020	0.0038	0.006	0.0013
42	0.089	0.15	1.00	2.30	0.44	0.34	0.060	0.068	0.0019	0.0031	0.004	0.0022
43	0.089	0.13	1.05	2.00	0.48	0.37	0.020	0.060	0.0014	0.0027	0.005	0.0017
44	0.100	0.11	0.90	2.30	0.43	0.47	0.060	0.070	0.0021	0.0024	0.006	0.0012
45	0.090	0.14	0.90	2.00	0.45	0.52	0.060	0.058	0.0007	0.0034	0.004	0.0010
46	0.079	0.11	1.14	1.80	0.43	0.33	0.010	0.045	0.0006	0.0036	0.006	0.0021
47	0.092	0.12	1.11	2.10	0.36	0.45	0.030	0.071	0.0007	0.0035	0.006	0.0012
48	0.096	0.13	0.76	1.80	0.67	0.48	0.020	0.086	0.0006	0.0040	0.006	0.0014
49	0.076	0.14	1.01	2.30	0.42	0.45	0.030	0.040	0.0012	0.0035	0.005	0.0013
50	0.079	0.15	1.04	2.30	0.44	0.55	0.010	0.085	0.0013	0.0031	0.005	0.0008
51	0.070	0.11	1.16	1.20	0.55	0.36	0.020	0.041	0.0011	0.0033	0.006	0.0024
52	0.070	0.12	1.03	2.00	0.45	0.50	0.030	0.053	0.0006	0.0034	0.004	0.0011
53	0.085	0.12	1.02	1.60	0.63	0.48	0.060	0.040	0.0019	0.0031	0.004	0.0022
54	0.090	0.12	0.92	1.38	0.48	0.32	0.030	0.046	0.0018	0.0027	0.006	0.0009

The blank indicates no intended addition.
The underline indicates that the underlined value is out of the range of the present invention.

[Table 1B]

No.	Chemical Composition (mass%, Remainder: Fe and Impurities)							α value (mass%)	β value	γ value (mass%)	fB (mass%)	Ceq. (mass%)
No.	Cu	Nb	Ti	Ca	Mg	REM	O	α value (mass%)	β value	γ value (mass%)	fB (mass%)	Ceq. (mass%)
1	0.44	0.005		0.0007			0.0012	1.36	12.06	1.01	0.0018	0.600
2	0.16	0.014			0.0024		0.0015	1.33	10.64	1.07	0.0008	0.583
3	0.19	0.009		0.0024	0.0014	0.0012	0.0032	1.49	10.66	1.31	0.0007	0.599
4	0.25						0.0018	1.42	10.62	0.75	0.0013	0.570
5	0.20	0.005	0.008				0.0010	1.37	10.24	1.22	0.0021	0.569
6	0.80	0.008				0.0011	0.0024	1.29	10.34	0.96	0.0021	0.576
7	0.30						0.0015	1.25	11.89	1.03	0.0018	0.602
8	0.03		0.007				0.0008	1.20	11.32	1.16	0.0007	0.591
9	0.55	0.015					0.0035	1.13	12.09	1.35	0.0017	0.613
10	0.60				0.0027		0.0015	1.37	11.11	1.18	0.0013	0.605
11	0.63	0.008		0.0010			0.0035	1.43	12.98	1.07	0.0018	0.618
12	0.23		0.004			0.0011	0.0019	1.50	11.69	1.16	0.0016	0.581
13	0.96			0.0004			0.0021	1.47	10.15	0.94	0.0018	0.581
14	0.31						0.0024	1.38	13.36	0.95	0.0017	0.620
15	0.30						0.0018	1.54	10.37	1.03	0.0019	0.588
16	0.48			0.0001			0.0021	1.61	12.54	1.00	0.0010	0.603
17	0.20						0.0019	1.48	9.26	0.82	0.0022	0.555
18	0.37			0.0012	0.0002	0.0017	0.0031	1.47	7.84	0.86	0.0012	0.523
19	0.40						0.0035	1.43	15.54	1.10	0.0014	0.668
20	0.42	0.005		0.0003	0.0011	0.0005	0.0009	1.38	16.54	1.28	0.0022	0.668
21	0.20	0.020	0.010				0.0023	1.14	10.70	1.66	0.0012	0.572
22	0.57	0.014	0.009				0.0015	1.32	12.69	1.78	0.0006	0.619
23	0.76	0.024	0.010				0.0014	1.12	10.02	1.93	0.0008	0.582
24	0.21				0.0005		0.0022	1.37	778	0.68	0.0009	0.523
25	0.20			0.0019			0.0032	1.36	12.97	0.93	0.0010	0.608
26	0.07						0.0026	1.50	11.67	0.97	0.0012	0.582
27	0.20			0.0011			0.0010	1.46	12.69	1.35	0.0016	0.618
28	0.26		0.005			0.0006	0.0031	1.35	11.76	1.08	0.0013	0.616
29	0.23		0.004			0.0014	0.0016	1.23	11.51	0.96	0.0011	0.609
30	0.23	0.019					0.0028	1.41	13.04	1.16	0.0021	0.619
31	0.23	0.006					0.0013	1.36	10.21	1.22	0.0011	0.603
32	0.30		0.004			0.0024	0.0025	1.24	10.66	1.11	0.0006	0.577
33	0.30				0.0025		0.0014	1.10	12.52	1.03	0.0000	0.603
34	0.73	0.005	0.010			0.0011	0.0031	1.28	10.24	1.41	0.0035	0.578
35	0.12		0.005				0.0014	1.36	10.19	1.10	0.0018	0.575
36	0.51	0.013	0.009			0.0022	0.0009	1.45	10.04	1.46	0.0016	0.575
37	0.66	0.008			0.0022		0.0024	1.27	11.01	0.96	0.0020	0.598
38	1.40	0.023	0.003				0.0009	1.37	11.77	1.36	0.0006	0.617
39	0.48	0.031		0.0007			0.0023	1.41	10.96	1.37	0.0010	0.594
40	0.47		0.020				0.0030	1.37	10.53	1.47	0.0015	0.596
41	0.53			0.0035			0.0019	1.37	11.62	0.75	0.0020	0.608
42	0.20		0.003		0.0036		0.0029	1.38	10.49	1.11	0.0019	0.590
43	0.21					0.0040	0.0013	1.35	10.97	1.05	0.0014	0.585
44	0.18		0.005				0.0045	1.37	11.79	1.08	0.0021	0.607
45	0.25				0.0012		0.0015	1.31	11.68	0.90	0.0007	0.596
46	0.93						0.0014	1.34	10.96	1.14	0.0006	0.605
47	0.10			0.0022			0.0023	1.40	11.22	1.11	0.0007	0.592
48	0.32		0.003				0.0028	1.45	12.09	0.87	0.0006	0.598
49	0.29						0.0030	1.42	11.35	1.01	0.0012	0.597
50	0.06						0.0023	1.47	12.98	1.04	0.0013	0.610
51	0.97		0.009			0.0012	0.0033	1.33	10.55	1.48	0.0011	0.594
52	0.37	0.005					0.0021	1.21	11.45	1.13	0.0006	0.596
53	0.27			0.0027			0.0033	1.22	12.88	1.02	0.0019	0.614
54	0.37	0.010	0.005		0.0018		0.0015	1.40	8.03	1.30	0.0018	0.526

[Table 2]

No.	Hot Rolling			Direct Quenching			Reheating and Quenching	Tempering
No.	Heating Temperature Before Rolling (°C)	Cumulative Rolling Reduction at 1,150°C to 900°C (%)	Plate Thickness After Rolling (mm)	Cooling Start Temperature (°C)	Cooling End Temperature (°C)	Average Cooling Rate at 700°C to 300°C (°C/sec)	Quenching Temperature (°C)	Tempering Temperature (°C)
1	1160	76	32	Ar₃ point or higher	300°C or lower	13		623
2	1100	66	48	Ar₃ point or higher	300°C or lower	11		629
3	960	58	50				903	660
4	1140	70	50				907	629
5	1160	64	36	Ar₃ point or higher	300°C or lower	11		628
6	1100	60	52				909	615
7	1020	60	60				892	659
8	1200	54	50	Ar₃ point or higher	300°C or lower	8	908	636
9	1200	78	36	Ar₃ point or higher	300°C or lower	10		642
10	1220	78	44	Ar₃ point or higher	300°C or lower	11		650
11	1060	64	60				912	655
12	1100	72	56				891	636
13	1020	56	50	Ar₃ point or higher	300°C or lower	8	910	626
14	980	54	44	Ar₃ point or higher	300°C or lower	11		655
15	980	64	50				891	627
16	980	68	48	Ar₃ point or higher	300°C or lower	11	892	640
17	1140	80	50				896	610
18	1060	64	28	Ar₃ point or higher	300°C or lower	11	906	624
19	1120	62	50				906	658
20	960	66	48	Ar₃ point or higher	300°C or lower	10	911	621
21	1200	80	50				910	648
22	960	68	50				900	643
23	1080	70	50				890	633
24	980	68	60				910	643
25	1200	58	50				902	654
26	960	56	50				914	647
27	980	74	50				903	656
28	1120	66	50				912	635
29	960	66	50				907	622
30	1140	76	50				909	630
31	1240	52	50				897	631
32	1140	52	50				903	655
33	1180	62	50				913	607
34	960	80	50				891	657
35	1080	80	50				898	647
36	1160	78	50				891	653
37	1000	54	50				902	627
38	1180	80	50				896	658
39	1060	72	50				906	656
40	1240	80	50				912	622
41	1220	80	50				907	631
42	1160	78	50				912	637
43	1140	72	50				915	641
44	1060	66	50				895	656
45	1280	58	50				912	624
46	920	76	50				912	659
47	1040	42	50				903	657
48	1200	80	38	lower than Ar₃ point	300°C or lower	12		640
49	1240	70	38	Ar₃ point or higher	higher than 300°C	6		649
50	1200	78	38	Ar₃ point or higher	300°C or lower	3		605
51	1100	58	36	Ar₃ point or higher	300°C or lower	11	852	622
52	1160	58	36	Ar₃ point or higher	300°C or lower	11	935	629
53	1020	62	40	Ar₃ point or higher	300°C or lower	10	895	680
54	1020	64	38	Ar₃ point or higher	300°C or lower	10	892	458

[Table 3A]

No.	Hardness of Base Plate		Microstructure of Base Plate	Characteristics of Base Plate
	Hardness of Base Plate		Microstructure of Base Plate	After Tempering (Before SR)					After SR
	Average (IIv)	Standard Deviation	Average Grain Size (µm)	Yield Strength (MPa)	Tensile Strength (MPa)	Yield Ratio (%)	Charpy Absorbed Energy vE (J) at -65°C at t/4 Position	-35°C CTOD δ (mm)	Yield Strength (MPa)	Tensile Strength (MPa)	Charpy Absorbed Energy vE (J) at -40°C at t/4 Position	-35°C CTOD δ (mm)
1	283	19	13.0	729	829	87	206	0.70	719	824	88	0.26
2	267	18	13.6	711	837	85	176	0.57	704	832	81	0.22
3	268	19	13.6	705	820	86	197	0.68	702	816	65	0.19
4	272	18	12.9	698	821	85	193	0.67	697	814	98	0.31
5	270	20	13.3	689	802	86	173	0.45	683	799	75	0.22
6	270	19	13.3	692	815	85	186	0.62	685	807	86	0.21
7	278	19	13.0	732	841	87	221	0.57	725	841	107	0.37
8	280	19	12.7	723	831	87	192	0.65	718	829	93	0.26
9	280	20	12.5	732	841	87	236	0.77	729	833	89	0.28
10	270	19	13.3	719	846	85	197	0.63	719	846	83	0.19
11	290	18	14.0	750	872	86	187	0.55	748	865	73	0.16
12	276	19	12.6	734	834	88	223	0.64	726	827	57	0.11
13	272	18	13.1	693	816	85	178	0.41	693	809	78	0.23
14	285	19	13.0	761	865	88	216	0.71	754	858	95	0.12
15	273	19	12.8	699	813	86	158	0.36	691	805	26	0.03
16	270	19	13.3	688	800	86	241	0.82	682	799	23	0.02
17	270	23	17.3	675	794	85	121	0.08	674	792	98	0.21
18	269	24	18.9	672	790	85	68	0.06	670	778	85	0.17
19	306	19	13.0	819	936	87	200	0.67	819	934	45	0.03
20	284	18	12.3	754	898	84	185	0.63	753	892	25	0.02
21	268	18	13.6	705	802	88	183	0.63	701	800	41	0.09
22	286	19	13.2	755	868	87	213	0.72	750	865	26	0.07
23	265	20	14.0	690	803	86	259	0.89	683	801	24	0.04
24	265	24	19.4	659	775	85	58	0.06	650	773	93	0.18
25	296	19	13.0	810	936	87	96	0.08	805	934	85	0.24
26	283	18	13.0	733	852	86	174	0.59	731	847	41	0.09
27	279	19	13.0	756	890	85	205	0.67	751	889	32	0.07
28	278	18	13.0	725	843	86	178	0.61	715	835	36	0.08
29	279	19	13.0	719	827	87	194	0.63	712	824	32	0.06
30	283	19	13.0	761	865	88	189	0.61	758	865	26	0.05
31	266	19	13.8	697	829	84	226	0.77	692	829	33	0.06
32	272	19	13.1	702	835	84	77	0.07	695	831	23	0.05
33	266	24	18.8	740	840	88	22	0.03	739	831	21	0.05
34	274	19	13.0	688	800	86	132	0.11	686	791	46	0.08
35	273	23	17.7	701	806	87	64	0.09	700	805	42	0.07
36	272	19	13.0	684	795	86	246	0.81	676	791	25	0.02
37	279	19	13.0	708	805	88	54	0.09	699	797	23	0.03
38	282	19	13.0	722	859	84	158	0.13	721	855	33	0.05
39	271	19	13.2	707	832	85	36	0.04	699	825	21	0.03
40	270	18	13.3	706	830	85	26	0.03	701	824	19	0.02
41	282	19	13.0	724	862	84	55	0.08	716	853	32	0.06
42	270	18	13.2	697	801	87	67	0.09	689	800	31	0.05
43	277	18	13.0	708	843	84	76	0.08	701	838	37	0.06
44	279	19	13.0	727	827	88	43	0.06	720	823	21	0.04
45	276	18	16.3	730	859	85	87	0.09	720	849	64	0.14
46	279	24	14.2	714	811	88	31	0.06	713	807	28	0.02
47	272	19	16.4	719	836	86	53	0.07	709	830	41	0.04
48	271	23	13.4	714	837	85	18	0.02	710	827	21	0.03
49	271	23	16.7	720	847	85	23	0.08	716	845	20	0.03
50	266	23	16.7	759	873	87	23	0.03	759	872	20	0.03
51	275	24	18.0	707	842	84	19	0.02	704	833	22	0.02
52	282	18	16.5	729	858	85	57	0.06	720	849	67	0.07
53	267	19	13.7	699	822	85	78	0.07	697	817	32	0.04
54	289	22	16.3	758	874	87	127	0.10	697	777	79	0.19

The underline indicates that the underlined value is out of the range of the present invention.

[Table 3B]

No.	Characteristics of Joint
	As weld			After SR
	Collection from Surface -65°C FL Charpy (J)	Collection from t/2 -65°C FL Charpy (J)	-35°C CTOD (mm)	Collection from Surface -40°C FL Charpy (J)	Collection from t/2 -40°C FL Charpy (J)	-35°C CTOD δ (mm)
1	116	140	0.25	113	98	0.18
2	111	131	0.23	127	102	0.21
3	111	132	0.22	97	84	0.14
4	107	129	0.22	134	115	0.24
5	109	125	0.21	108	82	0.13
6	111	128	0.24	116	99	0.20
7	121	139	0.27	112	92	0.16
8	115	138	0.23	105	89	0.16
9	116	129	0.24	111	90	0.17
10	110	131	0.24	121	94	0.16
11	127	153	0.28	103	82	0.12
12	118	133	0.24	104	77	0.11
13	104	123	0.21	128	106	0.22
14	126	150	0.28	115	91	0.15
15	107	128	0.21	50	24	0.03
16	108	122	0.21	39	23	0.02
17	38	58	0.02	129	107	0.22
18	26	44	0.02	116	95	0.19
19	143	160	0.33	48	27	0.03
20	120	143	0.27	48	25	0.02
21	111	129	0.22	48	28	0.03
22	127	141	0.29	42	24	0.02
23	103	127	0.19	36	19	0.02
24	21	32	0.02	136	109	0.20
25	43	56	0.02	46	26	0.02
26	68	86	0.09	46	25	0.02
27	122	143	0.27	46	26	0.02
28	119	139	0.26	40	24	0.02
29	78	92	0.11	42	25	0.02
30	67	91	0.08	38	21	0.02
31	54	76	0.04	42	28	0.02
32	42	69	0.02	47	23	0.02
33	18	44	0.02	40	15	0.02
34	108	131	0.21	43	25	0.02
35	26	51	0.02	40	25	0.02
36	103	130	0.21	38	19	0.02
37	34	47	0.02	41	18	0.02
38	83	108	0.15	48	29	0.02
39	31	58	0.02	35	18	0.02
40	26	51	0.02	33	20	0.02
41	45	62	0.01	48	35	0.02
42	57	72	0.06	67	42	0.02
43	52	79	0.03	58	37	0.02
44	35	55	0.02	35	18	0.02
45	54	73	0.07	61	45	0.06
46	76	89	0.03	56	39	0.03
47	98	120	0.04	96	74	0.05
48	56	73	0.04	70	49	0.03
49	46	67	0.02	92	65	0.05
50	67	89	0.04	79	64	0.03
51	45	61	0.02	54	38	0.02
52	79	102	0.06	86	65	0.05
53	96	112	0.05	113	87	0.04
54	28	43	0.03	129	108	0.18

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a steel plate that has excellent strength and low-temperature toughness, and also has excellent strength and low-temperature toughness after stress relieving annealing. The steel plate is suitable for a liquefied CO₂ transportation tank and has high industrial applicability.

Claims

A steel plate comprising, as a chemical composition, by mass%:
C: 0.07% to 0.11%;

Si: 0.10% to 0.15%;

Mn: 0.70% to 1.20%;

Ni: 1.00% to 2.50%;

Cr: 0.20% to 0.80%;

Mo: 0.20% to 0.80%;

V: 0.005% to 0.070%;

Al: 0.010% to 0.100%;

B: 0.0005% to 0.0030%;

N: 0.0015% to 0.0050%;

P: 0.006% or less;

S: 0.0030% or less;

Cu: 0% to 1.00%;

Nb: 0% to 0.030%;

Ti: 0% to 0.010%;

Ca: 0% to 0.0030%;

Mg: 0% to 0.0030%;

REM: 0% to 0.0030%;

O: 0.0040% or less; and

a remainder: Fe and impurities,

wherein an α value defined by Equation (1) is 1.00 to 1.50 mass%,

a β value defined by Equation (2) is 10.0 to 15.0,

a γ value defined by Equation (3) is 0.70 to 1.50 mass%,

a Ceq value defined by Equation (4) is 0.550 to 0.620 mass%,

a yield strength is 670 to 870 N/mm²,

a tensile strength is 780 to 940 N/mm²,

a Charpy absorbed energy at -65°C is 100 J or more,

in measurement of a hardness distribution at a pitch of 0.05 mm in 1 mm × 1 mm at a 1/4 thickness position, an average value of hardness at 121 measurement positions is 265 Hv to 290 Hv and a standard deviation is 20 or less, and

a plate thickness is 10 to 60 mm, $α = [C] + 6 \times [Si] + 100 \times [P]$ β = 0.65 × [C]^1/2 × (1 + 0.64 × [Si]) × (1 + 4.10 × [Mn]) × (1 + 0.27 × [Cu]) × (1 + 0.52 × [Ni]) × (1 + 2.33 × [Cr]) × (1 + 3.14 × [Mo]) $γ = [Mn] + 20 \times [Nb] + 36 \times [Ti]$ $Ceq = [C] + [Mn] / 6 + ([Cu] + [Ni]) / 15 + ([Cr] + [Mo] + [V]) / 5$

in Equations (1) to (4), [C] is a C content, [Si] is an Si content, [P] is a P content, [Mn] is an Mn content, [Cu] is a Cu content, [Ni] is an Ni content, [Cr] is a Cr content, [Mo] is an Mo content, [Nb] is an Nb content, [Ti] is a Ti content, and [V] is a V content (mass%), and for amounts of elements that are not contained, including amounts of elements incorporated as impurities, 0 is substituted.
The steel plate according to Claim 1,
wherein [fB] obtained by Equations (A) to (E) is 0.0003 mass% or more, $[fB] = [B] - 0.77 \times [fN]$ $[fN] = [N] - 0.29 \times [fTi] - 0.52 \times [fA 1]$ $[fTi] = [Ti] - 2 \times [fO]$ $[fA 1] = [A 1] - 1.125 \times [fO]$ $[fO] = [O] - 0.4 \times [Ca] - 0.66 \times [Mg] - 0.11 \times [REM]$ in Equations (A) to (E), [B] is a B content, [N] is an N content, [Ti] is a Ti content, [Al] is an Al content, [O] is an O content, [Ca] is a Ca content, [Mg] is an Mg content, and [REM] is an REM content (mass%), for amounts of elements that are not contained, including amounts of elements incorporated as impurities, 0 is substituted, and in a case where calculated values of [fN], [fTi], [fAl], and [fO] are less than 0%, 0 is substituted.
The steel plate according to Claim 1 or 2,
wherein, in a case where regions surrounded by grain boundaries having a crystal orientation difference of 15° or more, determined by performing crystal orientation analysis using an electron backscatter diffraction method, are defined as crystal grains, circle equivalent grain sizes of the crystal grains are defined as grain sizes and a value calculated by area-weighted averaging in which weighting is performed by an area for each crystal grain is defined as an average grain size, the average grain size at a 1/4 thickness position is 15.0 µm or less.
The steel plate according to Claim 1 or 2,
wherein, in a case where the steel plate is subjected to stress relieving annealing at a holding temperature of 600°C, a holding time of 2 hours, and a temperature rising rate and a temperature dropping rate of 55 °C/hr or slower in a temperature range of 425°C or higher, at a portion subjected to the stress relieving annealing, a yield strength is 670 to 870 N/mm² a tensile strength is 780 to 940 N/mm², and a Charpy absorbed energy at -40°C is 27 J or more.
The steel plate according to Claim 3,
wherein, in a case where the steel plate is subjected to stress relieving annealing at a holding temperature of 600°C, a holding time of 2 hours, and a temperature rising rate and a temperature dropping rate of 55 °C/hr or slower in a temperature range of 425°C or higher, at a portion subjected to the stress relieving annealing, a yield strength is 670 to 870 N/mm² a tensile strength is 780 to 940 N/mm², and a Charpy absorbed energy at -40°C is 27 J or more.